[eng] "I believe in the Holy Spirit which sanctifies the Church". This holy Church, throughout history, is simultaneously a community of sinners constantly called to conversion. As an earthly institution, it is also subject to fallibility and frailty (errors, divisions, moral failures). Therefore, according to Vatican II, the Church is sancta simul et semper purificanda (LG 8C), "Christ summons the Church to continual reformation as she sojourns here on earth. The Church is always in need of this, in so far as she is an institution of men here on earth" (UR 6A). On this condition only can it be the credible sign of the coming Kingdom of God. Thanks notably to someone like Yves Congar, Vatican II has managed to work with the paradox inherent in ecclesial holiness, which is both given and yet requires constant progress in its practice. How can we avoid ecclesial idealism? How can we foster realistic ecclesiology without losing sight of the theological mystery of the Church?